Effects of glucagon-like peptide-1 and exendins on kinase activity, glucose transport and lipid metabolism in adipocytes from normal and type-2 diabetic rats

Abstract

Several kinases have been implicated in the metabolic response of human and rat myocytes to glucagon-like peptide-1 (GLP-1), exendin-4 (Ex-4) and exendin-9 (Ex-9). We have investigated, in isolated rat adipocytes, the changes caused by GLP-1, Ex-4 and Ex-9 compared with those provoked by insulin or glucagon, upon the activity of phosphatidylinositol-3-kinase (PI3K), protein kinase B (PKB), p42/44 MAP kinases (MAPKs) and p70s6 kinase (p70s6k), and the participation of these kinases and protein kinase C (PKC) in their action upon 2-deoxy-d-glucose uptake, lipolysis and lipogenesis. The study was conducted in normal rats, and extended to a streptozotocin-induced type-2 diabetic model (STZ-rats). The participation of distinct kinases was estimated by using potential kinase inhibitors, including wortmannin, PD98059, rapamycin, H-7 and RO31–8220. In normal rat adipocytes, GLP-1 and both exendins share with insulin an increasing action upon the activity of all kinases studied (except PKB), PI3K, p44 and p42 MAPKs and possibly PKC, all being required for their stimulating effect upon glucose uptake. Ex-4 and Ex-9, like GLP-1 and insulin, have lipogenic action, while only Ex-4 shares with GLP-1 its lipolytic effect which is antagonized by Ex-9. MAP kinases and PKC seem to have an essential role in the GLP-1 and Ex-4 lipolytic action, as does PI3K in that of Ex-4. An increase in PI3K and MAPKs activity for the lipogenic effect of Ex-4, Ex-9 and GLP-1 are required, and in the case of Ex-4 and Ex-9, a stimulation of p70s6k activity is also needed. In cells from STZ-rats the magnitude of the above parameters was, in general, comparable to that in normal animals, with some exceptions: basal PI3K activity and lipogenesis were higher, GLP-1, Ex-4 and Ex-9 failed to modify basal lipogenesis but increased PKB activity, insulin failed to affect the activity of MAPKs and the insulin-induced glucose uptake was impaired. The impaired insulin effects upon some of the variables in the STZ-rat, distinct from those of GLP-1 and exendins, adds knowledge to the mechanism of the beneficial action of GLP-1 and Ex-4 in diabetic states.